In modern dense wavelength division multiplex (DWDM) systems, multiple carrier wavelengths to be transported on a common fiber are spaced apart by a fixed and standardized distance within the frequency spectrum. This approach is useful for network planning and management and for interoperation of equipment. However, the wavelength spacing is not exactly tuned to the needs of the transmission of each of the wavelengths, thus not making full use of the capacity of the fiber for data transmission.
For example, the transmission characteristics are different between different wavelengths, and different channel wavelengths from a source node may be for transmission to different destinations with different reach. For example, some wavelengths may be for transmission to a remote node at the end of the transmission fiber, whereas other wavelengths may be dropped at intermediate nodes or OADM (optical add/drop multiplex) sites.
The modulation of the data signal to be transmitted onto the carrier frequency allocated to a particular channel can be carried out in a number of ways. Typically, the modulation of digital signals onto a fixed frequency carrier will involve one or a combination of amplitude shift keying and phase shift keying. These modulation techniques provide modification to the amplitude or phase of the carrier to provide a representation of one or many digital 1s or 0s within each sample period.
Binary modulation techniques provide encoding of a single bit. However, multiple level modulation schemes are also possible, in which each encoding operation encodes a multiple bit word. In this case, the same signalling rate (i.e. number of modulated transmissions per second) results in a higher bit rate. This is, of course, at the expense of more complicated modulation and demodulation systems, and such modulation systems are inevitably more susceptible to noise and are therefore appropriate for shorter reach transmissions.
Typically, the modulation system employed is tailored to meet the most stringent requirements within the optical communications system. This inevitably results in some inefficiency.
It is apparent that different transmission systems could conceivably be used for different links within an optical network, so that the transmission is tailored to the specific requirement of the link. However, there is also the need to reduce inventory within the transmitters and receivers, and standardization of components is therefore clearly desirable.